The present invention relates to a charged particle beam apparatus that scans a specimen by irradiation of a charged particle beam to obtain secondary electrons from the specimen, and to an aberration correction value calculation unit therefor and an aberration correction program therefor.
A charged particle beam apparatus (for example, an electron microscope such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM)) necessarily uses a lens that utilizes an electric field or magnetic field in order to focus a charged particle beam. Various types of aberration occur inevitably in electric field lenses and magnetic field lenses. As a result, simple narrowing of a spot diameter of a charged particle beam by reducing the magnification of a lens does not lead to an image of good quality if aberration of the charged particle beam is large.
Consequently, many charged particle beam apparatuses incorporate an aberration corrector in order to obtain an excellent image. Usually, an aberration corrector comprises multipole lenses arranged in multiple stages, and generates an electric field or a magnetic field within the multipole lenses in order to remove aberration included in a charged particle beam that has passed through the inside of the lenses.
Such an aberration corrector, which uses four stages of multipole lenses, is disclosed in the following Non-patent Document 1, for example.
Non-patent Document 1: Nuclear Instruments and Methods in Physics Research, A363 (1995), pp. 316-325.
Further, there is a technique of detecting and correcting aberration of a charged particle beam apparatus, as disclosed in Japanese Un-examined Patent Application Laid-Open No. 2005-183085 (hereinafter, referred to as Patent Document 1), for example. In this technique, a just-focused (i.e. in-focus) image and a plurality of defocused images are acquired, each item of image data is subjected to Fourier transformation, the Fourier transformed defocused image data are divided by the Fourier transformed in-focus image data, and the obtained value is subjected to inverse Fourier transformation, to obtain beam profile data. Various aberrations are obtained on the basis of the beam profile data, and the various aberrations are removed by operating an aberration corrector according to the various aberrations. Further, Japanese Un-examine Patent Application Laid-Open No. 2001-068048 (hereinafter, referred to as Patent Document 2) discloses a technique of correcting astigmatism by using items of image data having different focus conditions.
However, when each aberration is obtained by the method described in Patent Document 1, high S/N (signal to noise) ratio image data should be obtained. Thus, there is a problem in that obtainment of each aberration requires a lot of time, and as a result, obtainment of aberration correction requires a lot of time.
The noise in question include noise generated in the course of generation of a charged particle beam and in the course of generation of secondary particles, and noise generated in a detector for detecting the secondary particles, an amplifier for amplifying the output of the detector, and the like. The former noise is stochastic noise concerning particles, and result from, for example, dispersion of the number of generated charged particles or dispersion of the number of generated secondary particles. The latter noise is generated by devices themselves, such as the detector or the amplifier.
In the technique of Patent Document 1, division of the Fourier transformed defocused image data by the Fourier transformed in-focus image data means multiplication of random noise included in the defocused image data by random noise included in the in-focus image data. As a result, the final obtained beam profile data include much noise. Thus, it is necessary to obtain high S/N image data, for example by slowing the scanning speed of charged particle beam.
Patent Document 2 discloses a technique of correcting astigmatism, but does not disclose a technique of correcting higher-order aberration (second order or higher aberration) such as coma aberration and star aberration.